Elizabeth would not oppose such an injunction -- and a moment's consideration making her also sensible that it would be wisest to get it over as soon and as quietly as possible, she sat down again, and tried to conceal, by incessant employment, the feelings which were divided between distress and diversion.

Mr. Collins proposal is already being discussed, so I choose to focus on Elizabeth's reaction. Elizabeth's emotions are divided. She really does not want to be proposed to, but since it is Mr. Collins she is also amused. Amusement is the first predominant emotions

The idea of Mr. Collins, with all his solemn composure, being run away with by his feelings, made Elizabeth so near laughing that she could not use the short pause he allowed in any attempt to stop him farther,

I think in retrospect she will be able to laugh at the whole of the proposal.
However, with his pompousity and refusal to be refused, Mr. C is just too aggravating to be laughed away at the time.